Numerous scientific, civilian, and military applications require both narrowband and broadband communications. In typical applications, space and/or weight are at a premium and multiple frequency operation is necessary. Under these circumstances, using multiple antennas or larger broadband antennas is not practical. The use of a single antenna would eliminate cross-talk problems typically affecting multi-antenna systems, especially critical in shipboard and aircraft systems.
When limited space is a factor and multiple frequency operation is necessary, reconfigurable antennas provide flexibility in operating frequency, bandwidth, and radiation pattern performance. To be reconfigurable, prior designs have implemented optoelectronic or microelectromechanical systems (MEMS) switches placed along the antenna for control and sampling of electrical signals. These devices are ideal for reconfiguring antennas to different lengths, allowing for multifunctioning of the antennas. In particular, there is a need to have broadband antennas that can be reconfigured into narrowband antennas with high gain or high directionality and back to broadband for some applications.
A prior art concept is depicted schematically in FIG. 1, where optoelectronic switches 12a, 12b, 14a, and 14b interconnect dipole antenna 20 with antenna segments 22a, 22b, 24a, and 24b. The activating light is provided via optical fibers 30, resulting in complete isolation of the optoelectronic switches 12a, 12b, 14a, and 14b. When the light sources 40 and 42 are in a non-emissive state, antenna segments 22a, 22b, 24a, and 24b are inactive and dipole antenna 20 has a length L with output frequency F1 at time t1. When light source 40 is placed in an emissive state, optoelectronic switches 12aand 12b are actuated, thereby activating antenna segments 22a and 22b to form a dipole antenna with length 2L and output frequency F2 at time t2. When light source 42 is placed in an emissive state, while light source 40 is also in an emissive state, optoelectronic switches 14a and 14b are actuated, thereby activating antenna segments 24a and 24b to form a dipole antenna with length 3L and output frequency F3 at time t3. The disadvantage of this system, however, is that the antenna effectively samples only one frequency at a time. During the time that this one frequency is being observed, all of the information transmitted or received at other frequencies is lost. Thus, there is a need for a variable length antenna that may be switched to allow fast sampling over an entire frequency range, providing the equivalent frequency coverage of a broadband antenna while maintaining the high efficiency of a narrowband antenna.